Although they can be applied to any desired semiconductor structures having a trench, the present invention and the problem on which it is based are explained on the basis of the fabrication of a trench capacitor.
Integrated circuits (ICs) or chips use capacitors for the purpose of charge storage. An example of an IC which uses capacitors for storing charges is a memory IC, such as for example a chip for a dynamic random access memory (DRAM). The charge state (“0” or “1”) in the capacitor in this case represents one data bit.
One type of capacitor which is customarily employed in DRAMs is a trench capacitor. A trench capacitor is a three-dimensional structure formed in the silicon substrate. The volume or capacitance of the trench capacitor can be increased by etching more deeply into the substrate. In this case, the increase in the capacitance in the trench capacitor does not enlarge the surface area of the memory cell which is taken up by the wafer at all.
A standard trench capacitor includes a trench etched into the substrate. This trench is typically filled with n+-doped polysilicon which acts as a capacitor electrode (also known as a storage capacitor). A second capacitor electrode (also known as a “buried plate”) is optionally formed by outdiffusion of n+-dopants from a dopant source into a region of the substrate which surrounds the lower section of the trench. An n+-doped silicate glass, such as an arsenic-doped silicate glass (ASG), in this case serves as the dopant source. A storage dielectric which contains nitride is customarily used to insulate the two capacitor electrodes.
A dielectric collar is produced in the upper region of the trench in order to prevent a leakage current from the capacitor terminal with the buried plate. The storage dielectric in the upper region of the trench, where the collar is to be formed, is removed before the latter is formed. Removing the nitride prevents a vertical leakage current along the collar.
It is also known to produce a sacrificial collar in the upper region of a deep trench in order to enable certain processes to be carried out in the lower region of the trench, the intention being that these processes should leave the upper region of the trench unaffected. This sacrificial collar is then removed again after these certain processes have been carried out, and finally the electrically insulating insulation collar is built up in the upper region of the trench.
The fact that known concepts for fabricating sacrificial collars or insulation collars cannot readily be scaled, since they are based on an undoped polysilicon filling which for small design dimensions has an increasing tendency to form long voids, has proven to be a problem of such concepts. These voids make it impossible to control the further procedure.